The torque reactors on the Space Telescope are similar to the device Jiff sketches. They use the conservation of angular momentum to simply rotate the telescope by rotating a spinning mass. Why? Because the exhaust of a reaction rocket motor would create a cloud of gas around the scope that would harm the image, and perhaps deposit on the optics. Of course, the "inertia" engine produces no thrust, it just re-points the direction of the total angular momentum of the scope system. Scroll down to "Pointing System" at this site:
http://www.gsfc.nasa.gov/gsfc/servic...pe_systems.htm